The processor on the S6 is better, also the 6p is ****ing giant. But on the other hand.. Stock android on the 6P..

Pros of s6:

Faster processor
Better camera(barely)

Cons:

Smaller battery than the 6p
Touchwiz has a nasty reputation.
I hear it has bad ram management
Wont receive as many updates as the nexus(does it have marshmallow yet?)